Automatic Landing Systems (i.e., autopilots) on conventional commercial airplanes receive guidance from a ground-based Instrument Landing System (ILS). In inclement weather, the integrity and continuity of the ILS transmissions are crucial to the safety of the airplane during the final phase of approach, touchdown, and roll-out. “Integrity” is the probability that the signals are not hazardously misleading. “Continuity” is the probability that the signals remain present and usable during the approach. The integrity is assured by a set of near-field and far-field monitors, ready to shut down the ILS should the ILS signals move outside allowed tolerances. The continuity of the signals is assured by a backup transmitter. The backup transmitter comes on-line if the primary transmitter fails or is shut down. In conventional systems, the ground station has the sole responsibility for ensuring the integrity and continuity of its own transmissions.
ILSs are only practical at airfields that have large incomes generated by commercial traffic or government finding because ILS equipment is costly due to initial purchase price and maintenance costs. Also, ILS signals are sensitive to local building construction and even vehicle movement. This sensitivity increases operating costs, because the ILS operators, such as the Federal Aviation Administration (FAA), must continually ensure each ILS is producing an accurate signal.
Global Positioning System (GPS) Landing System (GLS) has been provided as a replacement for ILS. GLS uses satellite signals that are present at no cost to airports or other authorities responsible for providing airplane approach information. In present GLSs, airplane position signals, determined from GPS signals sent by orbiting satellites, are augmented in the airplane by differential corrections (differential GPS) received from a local ground station. The differentially corrected GPS signals are referenced to an intended approach path received by the airplane from the same ground station. The ground station is also responsible for monitoring each satellite and providing airplanes with the integrity status of each satellite. The integrity and continuity of the received airplane position signals depend on the number of satellites in the airplane's field of view, the satellites' positions in the sky (their “geometry”), and the data received from the ground station. The airplane's on-board equipment must determine that the signals being received from satellites and ground station will provide a level of integrity and continuity compatible with the prevailing approach weather minimum for the duration of the approach about to be performed.
Even when the satellite geometry supports the required continuity and integrity, the signals received by airplanes are subject to environmental threats, such as electromagnetic interference (EMI) (both accidental and malicious), lightning and ionospheric scintillation (i.e., brown-outs associated with sunspot activity). There is also the threat of random satellite failures and satellites setting over the horizon. These threats can affect the reception of some or all of the available satellite signals, resulting in degradation or loss of guidance.
When satellite signals are interrupted, many seconds may be needed to re-acquire and track their signals. During GPS signal interruption, positional deviations from the desired flight path cannot be computed and provided to the autoland system by the GLS.
Some conventional autoland systems have the ability to coast solely on navigation data provided by the Inertial Reference Systems (IRS) for lateral and vertical flight control. Such inertial coasting is only possible for a period of 10 seconds. However, the GPS signal interruption can be as long as one or two minutes, which is much longer than the tolerable coasting time by autoland systems. For a CAT II/III autoland system operating in the GLS environment, it must provide continuous operations from Alert Height through rollout even during interruptions of GPS signals. As such, GPS signal interruptions for more than 10 seconds pose a problem in terms of continuity for the CAT II/III autoland systems.
Several methods of enhancing GLS for providing acceptable signals for autoland approaches have been proposed. One method is to enhance the satellite constellation by making use of more satellite systems, such as the Russian GLONASS system. This approach places an added burden on the airborne equipment and has complex political implications. Another method uses so-called “pseudolites,” ground-based transmitters, located on or near the airport, which mimic satellites by providing additional range information to the airplane. Similar to ILS, this approach is impractical, because it entails large equipment expenditures and maintenance costs in addition to those of the differential GPS ground station. Also, neither of these approaches adequately addresses the environmental and other threats described above, which may produce unreliable GLS data for an indefinite period of time.
One proposed solution is described by U.S. Pat. No. 6,178,363 entitled “Inertially augmented GPS Landing System” and assigned to the Boeing Company. The Boeing system utilizes an algorithm operating with the GPS GLS and IRS to achieve inertial coasting performance during GPS signal interruptions while limiting computational burden. There are limitations, though, to this algorithm. Accordingly, an alternative to the Boeing algorithm is needed.